Resolvers and encoders are two types of transducers that are used to determine the angular position of a rotating shaft. For example, these devices are often incorporated in a machine tool to determine the angular position of a rotating shaft. Each type of transducer produces an electrical output which must be decoded to determine the angular position and direction of rotation.
The resolver has a coil wound on a rotor and two coils wound on its stator at 90.degree. with respect to each other. The shaft of the resolver rotor is coupled to the rotating mechanism whose position is to be determined. A signal generator produces two sinusoidal electrical signals which differ in phase by 90.degree.. These signals are commonly referred to as sine and cosine signals. The sine and cosine signals are applied to the stator coils so as to induce a signal in the rotor coil. As the rotor coil rotates, the phase angle of the signal induced in the rotor coil shifts in relationship to the angular position of the rotor. Typically, the angular position is determined by comparing the rotor signal to one of the excitation signals either the sine or cosine signal, to detect the phase difference. The magnitude of the phase difference corresponds to the angular position of the rotor shaft. By comparing the derived angular positions at two points in time the direction and velocity of rotation can be determined.
Encoders are another type of angular position transducer which operate entirely differently from resolvers. An incremental encoder produces two pulsed output signals which are in quadrature. The direction of the encoder's rotation may be determined by detecting which of the two output signals leads the other one. Each output signal contains an integral number of pulses per revolution. The angular position is determined by counting the number of pulse transitions in both of the signals. A third output signal from the encoder provides an index pulse once per revolution which is used to reset the position counter to the zero degree position, for example.
Not only do resolvers and encoders operate differently, but different manufacturers produce encoders that perform differently. For example, the polarity of the encoder output signals vary from manufacturer to manufacturer. Specifically, one company's encoder may produce a high logic level index pulse whereas another encoder may produce a low level index pulse. In addition, the length of the index pulse may vary from one device to another. All of these variations among different types of transducers have heretofore required separate electrical circuits specifically designed to process the output signals from each type of resolver or encoder.
If one of these transducers is incorporated in a machine tool for example, a resolver cannot be used in place of an encoder or vice versa. Furthermore, different types of encoders cannot be substituted for each other without redesigning the electronic signal processing circuitry. Without going through such an expensive and time consuming redesign process, a machine tool manufacturer may be limited to purchasing the transducer from only one source during the lifetime of the machine design. The ultimate user of the machine is at a further disadvantage since if a transducer fails, he does not have the ability to redesign the processing circuitry. The user must replace the failed transducer with one that exhibits identical characteristics as the original transducer, regardless of the cost or availability.